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where
f
is the amount of shallowing between 0 and 1,
with typical values between 0.45 and 0.7 for
compaction-caused inclination shallowing (Bilardello
& Kodama 2010b ). At mid - latitudes inclination shal-
lowing may be as much as 15-20°, whereas the effect
can be 10° or less and is closer in magnitude to the
resolution of typical paleomagnetic data, making
the shallowing harder to detect, at high or low lati-
tudes. This was the case for the paleomagnetic data
from tectonostratigraphic terranes from western North
America or central Asia. In both cases the expected
paleolatitudes for the terranes, if they had not moved
with respect to the craton, were intermediate in mag-
nitude: 30-40°N for the Cretaceous Peninsular Ranges
terrane and near to 45°N for the central Asian sites
with anomalous paleomagnetic directions from late
Mesozoic to early Cenozoic sedimentary rocks. In
both cases, interpretation of the paleomagnetic data
without taking account of the effects of compaction-
caused inclination shallowing led to the misinterpreta-
tion that about 1000 - 1500 km (approximately 10 - 15 °
of paleolatitude) of northward tectonic transport had
occurred.
The problem of inclination shallowing also became
evident in the so-called Pangea B problem. Paleomag-
netic data from the latest Paleozoic and earliest Meso-
zoic requires an overlap between Laurasia and
Gondwana in their equatorial regions. Rochette & Van-
damme (2001) have suggested that this is the result of
inclination shallowing forcing both Laurasia and
Gondwana closer to the equator in paleogeographic
reconstructions than they actually were, thus requir-
ing the Pangea B reconstruction in which Gondwana
is shifted eastward with respect to Laurasia. A counter-
clockwise ' twist ' along the Tethys seaway is needed to
bring the continents into their Pangea A confi guration
later in the Mesozoic, although there is scant geological
evidence for this megashear zone (Van der Voo 1993;
Torcq
et al
. 1997). Inclination shallowing as an expla-
nation for the Pangea B problem would be reasonable
if the size of the inclination bias were greater than the
typical 95% confi dence limits for the apparent polar
wander path used for the paleogeographic reconstruc-
tion. In Torsvik & Van der Voo's (2002) analysis to
determine the highest-quality Gondwana paleopoles at
about 250 Ma, when the Pangea B overlap persists, 11
of the 12 paleopoles are from sedimentary units. Five
of these sedimentary units with paleopoles closest to
the mean Gondwana paleopole for 250 Ma were all
deposited at intermediate latitudes in India, Pakistan,
and South America (Kamthi and Mangli beds and
Panchet beds of India, Wargal and Chhidru Formation
of Pakistan, and the Amana Formation of South
America). In fact, Bilardello & Kodama (2010a) showed
that if the Carboniferous sedimentary paleopoles for
Gondwana are corrected for inclination shallowing
and compared to an inclination-corrected paleopole for
North America, then a Pangea A confi guration is pos-
sible (Fig. 1.4 ).
Reduction diagenesis
In organic-rich marine and lake sediments iron-sulfur
reduction diagenesis causes iron oxide minerals to dis-
solve and be replaced by a sequence of iron sulfi des,
some of which are ferromagnetic (Karlin & Levi 1983,
1985 ; Canfi eld & Berner 1987). The mineral at the end
of the sequence of iron sulfi des produced is pyrite
(FeS
2
), which doesn't carry any remanence but is para-
magnetic. Both pyrrhotite (Fe
1 - x
S) and greigite (Fe
3
S
4
)
are important ferromagnetic minerals formed during
reduction diagenesis and will contribute a secondary
chemical remanent magnetization (CRM) to the sedi-
ment. These secondary minerals are of course formed
at the expense of the primary depositional magnetic
mineral, magnetite, as it dissolves. Because the small-
est magnetic grains have the largest surface area to
volume ratio, they are preferentially destroyed in the
process. The magnetic grain size is therefore seen to
coarsen during reduction diagenesis and the magnetic
hardness, or coercivity, of the magnetic particles of the
sediment decreases.
One good example of the reduction diagenesis
process comes from Lake Ely in northeastern Pennsyl-
vania where the concentration of fi ne - grained bio-
genic magnetite is seen to decrease downcore between
30 cm and 75 cm depth in the sediment column in
organic - rich lake sediments with a loss - on - ignition
(LOI) of 25%. Secondary Fe sulfi de magnetic phases
are produced during the dissolution of the primary
biogenic magnetite. The evidence is however indirect,
mainly from SIRM/χ ratios (the ratio of saturation iso-
thermal remanent magnetization, the highest mag-
netization acquired by a sample in a strong magnetic
fi eld, to magnetic susceptibility) that are high when Fe
sulfi des are present (Peters & Dekkers 2003). The depth
of reduction diagenesis in marine sediments is inversely
dependent on the organic carbon fl ux and ranges from
as deep as several meters for very low fl uxes of milli-
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